This invention relates to a thin-type aluminum solid electrolytic capacitor using a single-plate aluminum foil and, more specifically, relates to a capacitor fabrication method that directly forms a metal plating layer without a pretreatment at a cathode portion and then forms a metal plating layer at an anode portion to thereby realize low impedance characteristics at high frequencies and achieve an increase in capacity with stacked layers.
In recent years, following miniaturization, speed-up, and digitization of electronic devices, there has been a strong demand for small-size, large-capacity, and low-impedance capacitors having excellent high-frequency characteristics also in the field of capacitors.
Capacitors that are used in a high frequency region have conventionally been mainly multilayer ceramic capacitors which, however, cannot satisfy need for reduction in size, increase in capacity, and reduction in impedance.
As large-capacity capacitors, there are electrolytic capacitors such as conventional aluminum electrolytic capacitors and tantalum solid electrolytic capacitors. Liquid or solid electrolyte used in those capacitors, for example, manganese dioxide, has a high resistivity value of 1 ω·cm to 100 ω·cm and therefore it has been difficult to obtain a capacitor having a sufficiently low impedance in a high frequency region.
In recent years, however, there have been developed solid electrolytic capacitors using a conductive polymer compound such as polypyrrole or polythiophen as solid electrolyte. As compared with the conventional solid electrolyte in the form of a metal oxide semiconductor such as manganese dioxide, the solid electrolyte in the form of the conductive polymer compound has a lower resistivity value of 0.01 ω·cm to 0.1 ω·cm while a resistivity value (ρ) of a used electrolyte is inversely proportional to a frequency. Therefore, the solid electrolytic capacitor using the conductive polymer compound having the small resistivity value as the solid electrolyte is widely used because the impedance value in a high frequency region can be suppressed to a lower value.
As one example of an aluminum solid electrolytic capacitor using a conductive polymer compound as a solid electrolyte, a flat-plate element structure will be described. An anodic oxide film layer is formed on the surface of a belt-shaped aluminum foil surface-roughened by etching or the like and an insulating resin body is formed at a predetermined portion for dividing into an anode portion and a cathode portion. Thereafter, a conductive polymer film is formed at a predetermined portion and then a graphite layer and a silver paste layer are formed on the conductive polymer film in the order named, thereby forming the cathode portion. Thereafter, this element cathode portion and an external cathode terminal are connected together by the use of silver paste. Since the anode portion divided by the insulating resin body is in the form of the aluminum foil which is unsolderable, a solderable metal plate is electrically connected thereto by ultrasonic welding, electric resistance welding, laser welding, or the like.
The foregoing silver paste layer formed on the conductive polymer film contains epoxy resin, phenol resin, or the like for providing curing and adhesive properties. As a result, there is a disadvantage in that the conductivity of the silver paste layer decreases to 1/10 to 1/100 of that of pure silver. Further, as described above, since the aluminum foil at the anode portion is unsolderable, it is necessary to electrically connect a solderable metal of a different kind by the foregoing method or the like.
Therefore, the process is complicated and, in order to achieve reduction in impedance and reduction in thickness, a totally new method invention is necessary in terms of the silver paste layer, the connection method for the metal of the different kind connected to the anode portion, and the like.
Further, in order to achieve a small-size, large-capacity, and low-impedance capacitor which is mounted on a board with a limited floor area, a stacked structure is required as described in Japanese Unexamined Patent Application Publication (JP-A) 2001-358039. However, it has become difficult to achieve reduction in impedance by the use of the conventional silver paste or metal plate due to the influence of reduction in thickness and conductivity.
Recently, as described in Japanese Unexamined Patent Application Publication (JP-A) 2004-87872, there are a method of implementing metal plating after applying graphite onto a conductive polymer film at a cathode portion and a method of implementing metal plating after forming a deposition film of noble metal on a conductive polymer film at a cathode portion. However, these methods each also generate an interfacial resistance in a pretreatment at the cathode portion, increase the thickness of a capacitor, and require more process steps and cost.